1. Field of the Invention
The present invention relates to an optical transmission system that transmits and receives an optical signal via an optical transmission line and particularly to an optical transmission system having an optical transmission path redundantly configured with an active system optical transmission line and a standby system optical transmission line.
2. Description of the Related Art
In recent years, with the growing use of the Internet, the introduction and development of a wavelength division multiplexing (WDM) optical transmission system have been made in which the system transmits optical signals having different wavelengths all at once to a single optical fiber. Currently the transmission speed per a wavelength is mostly from 2.4 Gbps to 10 Gbps. However, in order to meet the increasing demand for information transmission, a higher transmission speed of, for example, 40 Gbps is beginning to be adopted.
In a WDM optical transmission system, a time slot for one bit becomes shorter and the band of wavelengths of a modulating signal spectrum becomes larger when the transmission speed per a wavelength becomes faster. Thus, intersymbol interference is produced by a waveform distortion (pulse broadening), which becomes a factor for limiting a transmission distance.
Thus, a technique for performing dispersion compensation to wavelength dispersion accumulated by an optical fiber transmission line inside a relay amplification node is important. A technique of adjusting the entire amount of wavelength dispersion to be zero by using a dispersion compensating fiber (DCF) having an inversed amount of wavelength dispersion of an optical fiber is used.
With a conventional signal speed of at most 10 Gbps, optical signals can be received by performing dispersion compensation all at once by using a dispersion compensating optical fiber (DCF) on the optical signals multiplexed with wavelengths by a wavelength division multiplexing apparatus. However, raising the transmission speed to 40 Gbps extremely narrows down the range of dispersion tolerance of a light receiver. For example, the dispersion tolerance is approximately ±11600 ps/nm when the transmission speed is 2.4 Gbps and is approximately ±800 ps/nm when the transmission speed is 10 Gbps. When the transmission speed is 40 Gbps, the dispersion tolerance is extremely small, being approximately ±80 ps/nm. For this reason, in addition to performing dispersion compensation on wavelength-multiplexed signals all at once, it is necessary to perform dispersion compensation individually on each wavelength-divided optical signal by using a variable dispersion compensator (VDC) in the case where the transmission speed is at least 40 Gbps.
As the control method of a variable dispersion compensator, a method is known to measure the bit error rate of a received optical signal and to set the dispersion compensation value of a variable dispersion compensator so that the bit error rate becomes the smallest (for example, see patent document 1). It requires, for example, at least a few minutes to search the dispersion compensation value at which the bit error rate becomes the smallest when setting the dispersion compensation value based on the bit error rate.    [Patent document No. 1] JP 2002-208892
In wavelength division multiplexing optical transmission systems, lines are normally relieved, in case of trouble, by transmitting optical signals by using optical transmission lines redundantly configured with active system optical transmission lines and standby system optical transmission lines. More specifically, optical transmission systems are configured in such a manner that standby system optical transmission lines are used, in case of some trouble with currently-operated active system optical transmission lines.
A dispersion compensation method using dispersion compensating fibers is employed in wavelength division multiplexing optical transmission systems where the transmission speed is 2.4 Gbps or 10 Gbps. Thus, an individual adjustment of the amount of dispersion compensation for each wavelength is not required, allowing for high-speed switching from active system optical transmission lines to standby system optical transmission lines.
On the other hand, a dispersion compensation method using variable dispersion compensators in addition to dispersion compensating fibers is employed in wavelength division multiplexing optical transmission systems where the transmission speed is at least 40 Gbps. However, the bit error rates of optical signals that have passed through standby system optical transmission lines cannot be measured during the transmission and reception of the optical signals by using active system optical transmission lines. Therefore, optical transmission line switching requires time since, in the event a failure occurs in the active system optical transmission lines, the variable dispersion compensators must be readjusted while bit error rates are being measured after the optical transmission lines are switched to standby system optical transmission lines.